1. Field of the Invention
This invention relates to measuring acceleration and vibration using optical measurement by light interference.
2. Description of the Prior Art
The flexure or strain of an elastic disk is a well-known basis for measurements including acceleration and pressure, acceleration typically being measured by such flexure resulting from momentum forces acting on such a disk in a direction along its axis. The amount of flexure may be determined interferometrically, mechanically, piezoelectrically, or by changes in capacitance or resistance of elements associated with the disk. However, all strain measuring devices have various deficiencies such as limited sensitivity, high cost, limited maximum deflection, and errors due to operating conditions. Some errors due to operating conditions may be fundamental, such as the flexure of a disk due to acceleration being indistinguishable from flexure due to pressure differential. Other operating condition errors are caused by changes in dimensions, modulus of elasticity, index of refraction, and the like caused by temperature and pressure.
Interferometric measurements of strain can provide great resolution and, when carried out with an optical fiber, provide a simple and rugged sensor which requires low power, is immune to electromagnetic interference, and is adapted to remote sensing and to high data rates. Although optical fiber interferometric measurements of acceleration and pressure may be carried out directly with fibers of suitable construction, optical fibers themselves are relatively insensitive per unit length when used directly and are subject to errors due to ambient pressure, tension from acceleration, and the like. Interferometers having a single optical fiber sensing leg are particularly subject to error due to temperature caused variations of the leg length. It is evident that increasing leg length to provide greater sensitivity may proportionately increase errors due to operating conditions.
It is known to minimize operating condition errors by a "push-pull" arrangement of a pair of interferometer optical fiber legs such that a change in a measured variable shortens one leg and lengthens the other leg while both legs change length together with undesired variations due to temperature, pressure, or acceleration. However to be effective, interferometric rejection of such common mode errors requires that both legs be subject to exactly the same conditions.
It is also known to increase the sensitivity of fiber optic acceleration and pressure measurements by using the fibers indirectly, as by arranging an optical fiber wound element for strain due to the displacement or deformation of a primary force measuring element. For example, U.S. Pat. No. 4,893,930 discloses a mass supported between pairs of resilient, cylindrical mandrels each wound with an optical fiber which is one leg of an interferometer. In this arrangement, displacement of the mass causes lateral expansion of one mandrel of a pair and contraction of the other thereof so that the fibers of the pair function in the above-described push-pull manner. Such fiber optic arrangements are effective, but may be somewhat limited in sensitivity and have the push-pull optical fiber legs separated spatially and thermally.
High sensitivity and minimal spatial and thermal separation of push-pull fiber optic legs are provided in hydrophones disclosed in U.S. Pat. No. 4,959,539. In these hydrophones, each side of an elastic and circumferentially supported disk is wound with a flat spiral of optical fiber fixedly attached to the disk side. As a result of this construction, flexure of the disk shortens such a spiral on one side of the disk and lengthens such a spiral on an oppositely facing side. Such a disk may be mounted on a body so that an acoustic pressure differential to be measured exists across the disk, the spirals then being connected for push-pull operation as two legs of a fiber optic interferometer to provide an output corresponding to the flexure while substantially canceling errors due to pressure and temperature effects common to the legs. In one such hydrophone, a pair of the circumferentially supported disks and associated optical fiber spirals are mounted on opposite ends of such a body with the outer spirals connected as one interferometer leg and the inner spirals as another leg so that differences in the lengths of the legs due to acceleration induced flexure of the disks are canceled. This double disk arrangement also has twice the sensitivity of the single disk arrangement. Even greater sensitivity would be desirable if the cancellation of common mode errors is not decreased.